Move context menus out of experimental (#2134)

Now that the context menu code is on stable, move the sample out of
experimental.

Fixes https://github.com/flutter/samples/issues/2110

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

---------

Co-authored-by: Brett Morgan <brett.morgan@gmail.com>

context_menus/lib/email_button_page.dart

@@ -0,0 +1,94 @@
+import 'package:flutter/material.dart';
+import 'package:url_launcher/url_launcher.dart';
+
+import 'constants.dart';
+import 'is_valid_email.dart';
+import 'platform_selector.dart';
+
+class EmailButtonPage extends StatelessWidget {
+  EmailButtonPage({
+    super.key,
+    required this.onChangedPlatform,
+  });
+
+  static const String route = 'email-button';
+  static const String title = 'Email Button';
+  static const String subtitle = 'A selection-aware email button';
+  static const String url = '$kCodeUrl/email_button_page.dart';
+
+  final PlatformCallback onChangedPlatform;
+
+  final TextEditingController _controller = TextEditingController(
+    text: 'Select the email address and open the menu: me@example.com',
+  );
+
+  DialogRoute _showDialog(BuildContext context) {
+    return DialogRoute<void>(
+      context: context,
+      builder: (context) =>
+          const AlertDialog(title: Text('You clicked send email!')),
+    );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text(EmailButtonPage.title),
+        actions: <Widget>[
+          PlatformSelector(
+            onChangedPlatform: onChangedPlatform,
+          ),
+          IconButton(
+            icon: const Icon(Icons.code),
+            onPressed: () async {
+              if (!await launchUrl(Uri.parse(url))) {
+                throw 'Could not launch $url';
+              }
+            },
+          ),
+        ],
+      ),
+      body: Center(
+        child: SizedBox(
+          width: 300.0,
+          child: Column(
+            mainAxisAlignment: MainAxisAlignment.start,
+            children: <Widget>[
+              const SizedBox(height: 20.0),
+              const Text(
+                'This example shows how to add a special button to the context menu depending on the current selection.',
+              ),
+              const SizedBox(height: 40.0),
+              TextField(
+                maxLines: 2,
+                controller: _controller,
+                contextMenuBuilder: (context, editableTextState) {
+                  final TextEditingValue value =
+                      editableTextState.textEditingValue;
+                  final List<ContextMenuButtonItem> buttonItems =
+                      editableTextState.contextMenuButtonItems;
+                  if (isValidEmail(value.selection.textInside(value.text))) {
+                    buttonItems.insert(
+                        0,
+                        ContextMenuButtonItem(
+                          label: 'Send email',
+                          onPressed: () {
+                            ContextMenuController.removeAny();
+                            Navigator.of(context).push(_showDialog(context));
+                          },
+                        ));
+                  }
+                  return AdaptiveTextSelectionToolbar.buttonItems(
+                    anchors: editableTextState.contextMenuAnchors,
+                    buttonItems: buttonItems,
+                  );
+                },
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}